The present invention generally relates to aluminum-lithium alloys and methods for thermal treatment of aluminum-lithium alloys and, more particularly, to a method to increase the toughness of the aluminum-lithium alloy C458 at cryogenic temperatures.
Structural weight is a major issue for single use and reusable launch vehicles, for spacecraft, and other space vehicle structures used in the aerospace industry since weight reduction enables increased payload capability and reduced cost. It is well recognized that aluminum-lithium alloys possess lower density, good and often higher strength than conventional aluminum alloys, and provide higher modulus, and therefore, enable weight savings in space vehicle structures.
For the aerospace industry, the toughness of aluminum alloys and aluminum-lithium alloys at cryogenic temperatures is of special interest, for example, for the use of such materials in cryogenic tanks. Cryogenic tanks are used to hold liquid hydrogen, liquid oxygen, or any other liquid requiring cryogenic storage. Therefore, it is important for the alloy used in such tanks to have both a relatively high strength and high toughness at cryogenic service temperatures. Compared with conventional aluminum alloys such as 2219, which is widely used in space vehicles for cryogenic tanks as well as unpressurized structures, aluminum-lithium alloys possess an attractive combination of lower density and higher modulus along with higher mechanical properties than non-lithium containing aluminum alloys commonly used for cryogenic tanks. The toughness of conventional aluminum alloys such as 2219 and 2014 is higher at cryogenic temperatures compared to that at room temperature. The ability to achieve higher toughness and strength at cryogenic temperatures enables a structural proof test for the cryotank, for example, to be conducted more inexpensively at room temperature than at cryogenic temperatures. In this case a successful room temperature proof test ensures that neither strength-overload-induced nor toughness-limited-induced failure will occur at cryogenic service temperatures. The toughness of aluminum-lithium alloys at cryogenic temperatures often remains the same as the toughness at room temperature or is reduced when the material is processed and heat treated using conventional single-step aging. If the toughness of an aluminum-lithium alloy is lower at cryogenic temperatures, the performance of acceptance testing at the cryogenic temperature has to be considered, which is a very expensive and involved approach.
A prior art aluminum-lithium alloy 2195 (Al-1.0 Li-4.0 Cu-0.4 Mg-0.4 Ag-0.12 Zr) with a low lithium content of nominally 1.0 wt. % has already been successfully used for the Space Shuttle external tank. As is well known, alloy processing and particularly the aging treatment, has a significant influence on the strength-toughness combinations and their dependence on the service environments for all aluminum-lithium alloys. A prior art two-step aging procedure developed for the aluminum-lithium alloy 2195 showed that the cryogenic toughness was improved when a low-temperature aging step was followed by a higher-temperature aging step. The purpose of the initial low-temperature aging is to provide nucleation sites within the matrix and to minimize the volume fraction of the strengthening precipitates at the sub-grain and grain boundaries, while the second step is necessary to increase the size of the precipitates to provide reasonable strength levels. These prior art heat treatment methods to improve the cryogenic toughness of the aluminum-lithium alloys were limited to the alloy 2195 and entailed processing conditions such as controlled heating rates or durations that may not be practical in the industry.
While a higher lithium content in aluminum-lithium alloys results in more weight savings, an aluminum-lithium alloy C458 (Al-1.8 Li-2.7 Cu-0.3 Mg-0.08 Zr-0.3 Mn-0.6 Zn) with a lithium content of nominally 1.8 wt. % and similar alloys from this family of alloys, such as C460 and C47A, have been shown to provide the greatest benefit for the aerospace industry. The composition and processing of alloy C458 and similar alloys in this family has been carefully tailored to increase the toughness and reduce the mechanical property anisotropy of the earlier generation alloys such as 2090 and 8090, which have a lithium content above 2.0 wt. %. Therefore, aluminum-lithium alloy C458 is considered as a material to be used for future space vehicle structures. To achieve the T8 temper material characteristics, alloy C458 in the T3 condition (achieved by solution heat treatment at an elevated temperature, quenching to room temperature and cold working, for example by stretching) is typically aged at 300° F. for a duration of twenty-four hours using conventional single-step aging. This prior art single-step aging process applied to alloy C458 results in a reduction of the toughness at cryogenic temperatures over that at room temperature of about 10%. Consequently, if the aluminum-lithium alloy C458 were to be used to build a cryogenic tank after the single step aging was conducted, expensive structural proof testing of the tank at cryogenic temperatures would be necessary to ensure proper strength and toughness parameters.
Prior art further includes, for example, U.S. Pat. No. 4,861,391 issued to Rioja et al., U.S. Pat. No. 4,812,178 issued to Dubost, and U.S. Pat. No. 4,648,913 issued to Hunt, Jr. et al., all disclosing aluminum based alloys containing lithium as well as heat treatment methods. U.S. Pat. No. 4,861,391 issued to Rioja et al. further discloses a two-step aging method for aluminum-based alloys; however the first aging step conducted at lower temperatures can take up to several months, which is inconvenient and expensive. Since a heat treatment process needs to be optimized for each alloy individually depending on the content of chemical elements in this alloy, none of the disclosed prior art heat treatments can be easily applied to the processing of the aluminum-lithium alloy C458. Further, none of the prior art patents recommends the disclosed aging procedures for application on alloy C458 or similar alloys.
There has, therefore, arisen a need to increase the toughness at cryogenic temperatures of the aluminum-lithium alloy C458 to enable its use for cryogenic tanks in space vehicles, and thereby, reducing the weight of the cryogenic tanks, increasing the payload capabilities, and reducing cost per launch. There has further arisen a need for the development of a heat treatment scheme for aluminum-lithium alloy C458 and similar alloys of this family that results in an increased toughness of the alloy C458 at cryogenic temperatures compared to room temperature. There has also arisen a need to develop a method for an aging treatment for alloy C458 that can be easily practiced in the manufacturing process, that does not involve impractical heating rates or durations, and that does not degrade other material properties. There has further arisen a need to provide specific sets of times and temperatures to age the aluminum-lithium alloy C458 to the T8 temper that result in higher toughness at cryogenic temperatures compared to room temperature, so that an optimized aging cycle can be selected during the manufacturing process.
As can be seen, there is a need for a method to increase the toughness of aluminum-lithium alloy C458 at cryogenic temperatures. Also, there is a need for an aging treatment for alloy C458 that increases the toughness at cryogenic temperatures and that can be easily practiced in the manufacturing process. Moreover, there is a need for specific sets of times and temperatures to age aluminum-lithium alloy C458 to the T8 temper that result in higher cryogenic toughness.